This invention relates to cooling devices in general and more particularly to an improved device for cooling a magnet system, especially in a nuclear spin tomography system.
Cooling devices for magnet systems such as are used in nuclear spin tomography apparatus, where the magnet system comprises several disc-shaped magnet coil windings which are made from ribbons of normal conducting material and are connected at their end faces over a large area to cooling elements in a thermally conducting manner and can be cooled by a coolant flowing under forced flow in its coolant lines are know. Such a cooling device is provided for a magnet system such as is indicated in the journal "Computer Tomography", Vol 1, (1981), pages 2 to 20 and, in particular, page 6.
In the field of medical diagnostics, image forming methods have been developed in which an image similar to an X-ray tomogram is constructed by computer or measurement analysis of integral resonance signals of nuclei such as protons from the spatial spin density and/or relaxation time distribution of a body to be examined. The corresponding method is called nuclear spin tomography or nuclear magnetic resonance tomography or also zeugmatography ("Nature", Vol. 242, 1973, pages 190 and 191). For nuclear spin tomography systems (Nuclear Magnetic Resonance Tomography Systems), a strong base field, on which the magnitude of the nuclear resonance signal depends, and which must meet stringent requirements as to its homogeneity, is desired. Thus, a corresponding magnet system should have a field deviation of less than 50 ppm in a spherical volume with a diameter of about 50 cm.
The magnetic base field of such a magnet system is generally generated by four or more rotationally symmetric coil windings which are made of normally conducting, electrically highly conductive material for field strengths of up to about 250 mT. For the design of these windings discs, known as Bitter coils, or tubular, internally cooled hollow conductors or a wide metal ribbon are available. If metal ribbon which may consist, for instance, of copper or aluminum, is used, high precision is combined with relatively low manufacturing costs. In the magnet system which can be seen from the literature reference mentioned at the outset, its four coil windings are made of such a metal ribbon of aluminum.
Since the electric power required for such coil windings for the mentioned field strength conditions is quite considerable and is converted practically completely into heat, Joule power in the order of 100 kW must be removed, at least in part, by appropriate cooling measures. The individual coil windings must not be excessively deformed so as not to degrade the homogeneity. Also temperature must not exceed certain limits in order to ensure technical safety, e.g., so to not destroy electric insulation. The requirement, therefore, exists that the temperature of the coil windings be kept highly stable, since otherwise magnetic field variation in space and time which degrade the image quality in nuclear spin tomography could occur.
The coil windings of the known magnet system have low thermal conductivity in the radial direction because, for instance, 100 to 300 turns of the wide metal ribbon are spaced from each other by a corresponding number of thin insulating layers. Effective cooling is, therefore, possible only from the end faces. In cases where the coil windings are used for nuclear spin tomography, such cooling measures should take only little space perpendicular to the end face so as not to collide with coil windings which are relatively closely adjacent. Furthermore, these measures must not protrude into the radial inside space since this space is required for gradient coils, high frequency coils and the body to be examined.
For cooling the individual coil windings of the known magnet system, a large, washer-shaped plate of aluminum is provided at both end faces of each winding; this plate contains pressed-in copper tubes through which water is conducted as the cooling medium under forced flow. Each plate with its tubular coolant lines, therefore, represents a cooling element. The two cooling elements of each winding are held on the respective end faces by means of mutual threaded connections. The thermal contact between the cooling element to the winding must be accomplished here via a permanently plastic compound, since cementing would tear because of the high temperature stresses between the end faces and the respective cooling element. The thickness of this compound, however, must be chosen relatively large so that the thermal resistance of this compound is accordingly high. With this fixation, the cooling elements can also travel on the winding so that the adjustment of the individual windings changes accordingly.
It is an object of the present invention to improve the cooling device described above in such a manner that the above-described difficulties are at least largely eliminated, i.e., an effective and secure cooling of the coil windings of metal ribbons is assured, so that these coil windings meet the requirements of nuclear spin tomopgraphy systems.